The present invention relates to methods and apparatuses for generating three-dimensional images, and in particular to devices employing light emitting sources to generate three-dimensional holograms.
Holography is an application of laser technology, best known for its ability to reproduce three-dimensional images. Early holography was limited to using film to record intensity and phase information of light incident on the scene.
More specifically, the principle of operation of film holograms or xe2x80x9cstereoscopic photographyxe2x80x9d is that the film records the interference pattern produced by two coherent beams of light, i.e., xe2x80x9crecording beamsxe2x80x9d. One recording beam is scattered from the scene being recorded and one recording beam is a reference beam. The interference patterns recorded on the film encode the scene""s appearance from a range of viewpoints. Depending on the arrangement of the recording beams, and therefore the reconstructing and reconstructed beams, with respect to the film, the hologram may be a transmission-type or reflection-type hologram.
For a transmission-type hologram, reconstructing the holographic image is accomplished by shining one of the recording beams, as a xe2x80x9creconstructingxe2x80x9d beam, through the developed hologram. By diffraction, the recorded interference pattern redirects some of the light to form a replica of the other recording beam. This replica beam, the xe2x80x9creconstructedxe2x80x9d beam, travels away from the hologram with the same variation in phase and intensity of the original beam. Thus, for the viewer, the reconstructed wavefront is indistinguishable from the original wavefront, including the three dimensional aspects of the scene.
Holography differs from stereoscopic photography in that the holographic image exhibits full parallax by affording an observer a full range of viewpoints of the image from every angle, both horizontal and vertical, and full perspective, i.e., it affords the viewer a full range of perspectives of the image from every distance from near to far. As such, a hologram contains a much higher level of visual and spatial information as compared to a stereoscopic image having the same resolution. In the same manner that a two-dimensional visual image can be represented in a two-dimensional array of picture elements, or xe2x80x9cpixels,xe2x80x9d a holographic image is often embodied in a three-dimensional array of volume picture elements, or xe2x80x9cvoxels.xe2x80x9d A holographic representation of an image thus provides significant advantages over a stereoscopic representation of the same image. This is particularly true in medical diagnosis, where the examination and understanding of volumetric data is critical to proper medical treatment.
While the examination of data that fills a three-dimensional space occurs in all branches of art, science, and engineering, perhaps the most familiar examples involve medical imaging where, for example, Computerized Axial Tomography (CT or CAT), Magnetic Resonance (MR), and other scanning modalities are used to obtain a plurality of cross-sectional images of a human body part. Radiologists, physicians, and patients observe these two-dimensional data xe2x80x9cslicesxe2x80x9d to discern what the two-dimensional data implies about the three-dimensional organs and tissue represented by the data. The integration of a large number of two-dimensional data slices places great strain on the human visual system, even for relatively simple volumetric images. As the organ or tissue under investigation becomes more complex, the ability to properly integrate large amounts of two-dimensional data to produce meaningful and understandable three-dimensional mental images may become overwhelming.
Other systems attempt to replicate a three-dimensional representation of an image by manipulating the xe2x80x9cdepth cuesxe2x80x9d associated with visual perception of distances. The depth cues associated with the human visual system may be classified as either physical cues, associated with physiological phenomena, or psychological cues, which are derived by mental processes and predicated upon a person""s previous observations of objects and how an object""s appearance changes with viewpoint.
The principal physical cues involved in human visual perception include: (1) accommodation (the muscle driven change in focal length of the eye to adapt it to focus on nearer or more distant objects); (2) convergence (the inward swiveling of the eyes so that they are both directed at the same point); (3) motion parallax (the phenomenon whereby objects closer to the viewer move faster across the visual field than more distant objects when the observer""s eyes move relative to such objects); and (4) stereo-disparity (the apparent difference in relative position of an object as seen by each eye as a result of the separation of the two eyes).
The principal psychological cues include: (1) changes in shading, shadowing, texture, and color of an object as it moves relative to the observer; (2) obscuration of distant objects blocked by closer objects lying in the same line of sight; (3) linear perspective (a phenomenon whereby parallel lines appear to grow closer together as they recede into the distance); and (4) knowledge and understanding that is either remembered or deduced from previous observations of the same or similar objects.
The various psychological cues may be effectively manipulated to create the illusion of depth. Thus, the brain can be tricked into perceiving depth which does not actually exist. Physical depth cues are not subject to such manipulation; the physical depth cues, while generally limited to near-range observation, accurately convey information relating to an image. For example, the physical depth cues are used to perceive depth when looking at objects in a small room. The psychological depth cues, however, must be employed to perceive depth when viewing a photograph or painting (i.e., a planar depiction) of the same room. While the relative positions of the objects in the photograph may perhaps be unambiguously perceived through the psychological depth cues, the physical depth cues nonetheless continue to report that the photograph or painting is merely a two-dimensional representation of a three-dimensional space.
Stereo systems depend on image pairs each produced at slightly different perspectives. The differences in the images are interpreted by the visual system (using the psychological cues) as being due to relative size, shape, and position of the objects and thus create the illusion of depth. A hologram, on the other hand, does not require the psychological cues to overrule the physical depth cues in order to create the illusion of a three-dimensional image; rather, a hologram produces an actual three-dimensional image.
Conventional holographic theory and practice teach that a hologram is a true three-dimensional record of the interaction of two beams of coherent, i.e. mutually correlated light, in the form of a microscopic pattern of interference fringes. More particularly, a reference beam of light is directed at the film substrate at a predetermined angle with respect to the film. An object beam, which is either reflected off of or shines through the object to be recorded, is generally normally (orthogonally) incident to the film.
The reference and object beams interact within the volume of space occupied by the film and, as a result of the coherent nature of the beams, produce a standing (static) wave pattern within the film. The standing interference pattern selectively exposes light sensitive elements within the photographic emulsion making up the film, resulting in a pattern of alternating light and dark lines known as interference fringes. The fringe pattern, being a product of the standing wave front produced by the interference between the reference and object beams, literally encodes the amplitude and phase information of the standing wave front. When the hologram is properly re-illuminated, the amplitude and phase information encoded in the fringe pattern is replayed in free space, producing a true three-dimensional image of the object.
Conventional holographic theory further suggests that a sharp, well-defined fringe pattern produces a sharp, bright hologram, and that an overly strong object beam will act like one or more secondary reference beams causing multiple fringe patterns to form (intermodulation) and diluting the strength of the primary fringe pattern. Accordingly, holographers typically employ a reference beam having an amplitude at the film surface approximately five to eight times that of the object beam to promote the formation of a single high contrast pattern within the interference fringe pattern and to reduce spurious noise resulting from bright spots associated with the object. In general, the resolution of the fringe pixel density determines the resolution of the final image.
Since known holographic techniques generally surround the recording of a single hologram or, alternatively, up to two or three holograms, within a single region of the emulsion making up the film substrate, the stated objective is to produce the strongest fringe pattern possible to ensure the brightest holographic display. Accordingly, holographers typically attempt to expose a large number of photosensitive grains within the film emulsion while the object is being exposed. Since every point within the holographic film includes part of a fringe pattern that embodies information about every visible point on the object, fringe patterns exist throughout the entire volume of the film emulsion, regardless of the configuration of the object or image which is the subject of the hologram.
As a consequence of the above, the creation of strong, high contrast fringe patterns necessarily results in rapid consumption of the finite quantity of photosensitive elements within the emulsion, thereby limiting the number of high contrast holograms that can be produced on a single film substrate to two or three. Some holographers have suggested that as many as 10 to 12 different holographic images theoretically may be recorded on a single film substrate; superimposing more than a small finite number of holograms has generally not been considered possible in the context of conventional hologram theory.
Known holographic display methods are useful primarily for the display of static images. Additionally, known holographic display devices are useful primarily for the display of recorded images.
Accordingly, there remains in the field of holographic projection a need for a display method able to display moving images. Additionally, there remains in the field a need for a display method able to display real-time computer-generated, rather than pre-recorded, three-dimensional images.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
The present invention relates to a method and apparatus for display of three-dimensional images. Although methods have been developed for display of three-dimensional images, numerous limitations have been identified in connection with prior holographic display devices. Specifically, known holographic display methods are useful primarily for the display of static images. Additionally, known holographic display devices are useful primarily for the display of recorded images.
There remains in the field of holographic projection a need for a display method able to display moving images. Additionally, there remains in the field a need for a display method able to display computer-generated, rather than prerecorded images. The inventors of the present invention have recognized that known holography methods, when used in combination with modern image-processing algorithms and recent advances in digital light processing technology, can be used to create three-dimensional moving holograms that can be generated and modified xe2x80x9con the flyxe2x80x9d by a computer. Alternately, the interference patterns corresponding to the desired three-dimensional holograms may be pre-computed and recorded on a storage medium, and later played back in real time.
In the present invention, a time-dependent computed image or virtual model of a real object is stored in, or generated by, a computer or dedicated digital signal processor (DSP). The image or model is then converted by the computer or DSP into its Fourier, or holographic, transform. The holographic transform is displayed on a light modulation device that is illuminated by one portion of a coherent light emission. The remaining portion of the same emission is combined with the holographic transform at a plane to create a three-dimensional image.
Certain embodiments of the present invention employ a digital micro-mirror device for light modulation. Digital micro-mirror devices have an advantage over other known light modulation devices such as liquid crystal displays (LCDs) owing to the fact that micro-mirrors preserve phase coherence of the light, whereas LCDs do not. Because of the high frequencies at which micro-mirrors can be moved, the device of the present invention allows for the creation and display of real-time, three-dimensional moving holograms.
In certain devices embodying the present invention, three-dimensional visual data can stream at full video rate without the necessity of higher bandwidth because the data representing the holographic transforms takes only as much bandwidth as normal two-dimensional video. Similarly to two-dimensional video, an increase in the resolution of the digital micro-mirror device increases the resolution of the three-dimensional images displayed.
In certain embodiments, the projection device may display in multiple colors through the use of a multi-mode laser or multiple lasers. Applications for the projection device of the present invention include next-generation television and movie projection, three-dimensional scientific workstations, haptics, interactive volumes, and three-dimensional robotic control displays.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.